Various four-ball test machines have been developed to evaluate the friction and wear characteristics of lubricants such as motor oils, hydraulic oils, cutting fluids, greases, solid lubricants or the like. Existing four-ball testers can also be used to evaluate the friction wear characteristics of bearing materials. In this case, the test balls themselves would be made of the material to be tested, and the same lubricant fluid would be utilized for the various varying materials to be tested. Known four-ball testers include a test cup with three steel balls positioned inside the test cup. The steel balls are held very tightly together by a conical ring that is secured via threaded fasteners to lock the balls in place. A typical four-ball test machine utilizes three steel balls that are ½ inch in diameter. A test fluid is placed inside the cup covering the three balls. With reference to FIG. 1, the test cup 2 is then positioned in a four-ball tester 1. The prior art four-ball tester illustrated in FIG. 1 includes a heater block 3 to heat the test cup 2 and fluid. As described in more detail below, a thrust bearing 4 supports heater block 3 on support column 5. Weights 7 are supported by lever arm 6 which pivots about a pivot 8 to push upwardly on the support column 5. A rotating shaft or spindle 9 has a fourth test ball at the end thereof (not shown) that contacts the three test balls in the test cup 2. The shaft 9 is connected to an electric motor 11 via a belt 10 for powered rotation of the shaft 9.
The amount of force pushing the three balls in the test cup 2 against the ball on the rotating spindle 9 can be varied by changing the weights 7. Typical lever arms 6 have a 10-to-1 ratio such that a four-kilogram weight 7 causes the bottom three balls in the test cup 2 to push up against the rotating top ball on spindle 9 with a force of 40 kg.
During a typical four-ball wear test 3 new test balls are placed into a clean test cup 2 and clamped securely in place above the heater block 3. The test fluid is then added to the test cup 2, and a new test ball is secured to the shaft 9. Shaft 9 includes a chuck (not shown) that secures the upper test ball to the shaft 9. During the test, the heater block 3 is turned on, and the temperature of the test fluid in the test cup 2 is monitored. When the test fluid reaches the test temperature, the load is applied and the motor 11 is actuated to thereby rotate the spindle 9.
A common ASTM test method utilizes a load of 40 kg, and is run at 1200 rpm at 75° C. for one hour. At the end of the test run the motor is stopped, the heater is turned off, the load is removed, and the test cup 2 is taken off the tester 1.
The thrust bearing 4 under the heater 3 allows the heater 3 and test cup 2 to rotate freely and also to move horizontally because one of the races of the thrust bearing 4 is replaced with a flat hard steel surface. With this arrangement, the cup 2 can freely move in any direction and center itself under the top rotating ball that is secured to the shaft 9. The top ball contacts each of the three bottom balls in the test cup 2. Upon rotation of the top ball, a force is transmitted to the test balls in the test cup 2, tending to rotate the test cup 2. However, a load cell 12 is contacted to the heater block 3 via a small chain 13 that prevents rotation of the heater block 3 and test cup 2. The load cell 12 is utilized to measure the rotational force acting on the test cup 2, and to thereby calculate the coefficient of friction.
Upon completion of the test, the test cup is removed from the tester 1, and the test lubricant is drained. The cup 2 with the three bottom test balls still locked in place is placed under a microscope to measure the wear scars. The wear scars result from the top ball rotating against each of the bottom three balls under the test load. The wear scars are measured for each of the bottom three balls. In general, each of the three bottom balls will have a wear scar that is very similar in size and shape to the other two lower test balls. A measurement is made with a microscope of each wear scar diameter, in both the vertical and horizontal direction. A total of six measurements are taken, two for each ball, and then the average of the six readings is considered to be the wear scar diameter for a given test fluid under a specific test method.
The torsional forces that have been measured with the load cell 12 can also be averaged out over the test period and can be used to calculate the average coefficient of friction for a given test fluid under specific test conditions. Two characteristics of a lubricant can be determined in this way: 1) the coefficient of friction of the fluid; and 2) the amount of wear that occurred in the presence of the test fluid under given test conditions.
Upon completion of a test utilizing a first test fluid, another lubricant or a modified formula of the same lubricant, can be run under the same test conditions, and the friction wear characteristics can be compared with those of the first lubricant.
As discussed above, the four-ball test provides information concerning: 1) the average wear-scar diameter; and 2) the average coefficient of friction, or a full friction trace for the test duration. The type of signal processing system attached to the tester will determine whether the average coefficient of friction or a full friction trace is produced by the test.
One frequent use for four-ball testers is as a quality control check for hydraulic fluids. Such tests commonly utilize a 40 kg load at 1200 rpm and 75° F. with a one hour duration. Other applications for four-ball tests include quality control tests for automatic transmission fluids. Four-ball tests are also used for quality control for many other automotive or other fluids such as brake fluids, motor oils, gear oils, torque fluids, mineral based cutting oils, water based cutting oils, and other fluids. A variety of fluids or semi-fluids for which wear and friction value are of importance can be tested using four-ball friction wear testing. The wear scar diameter and friction value can also be utilized to determine if a material has a proper composition. In such testing, the test balls are made of the material to be tested, and a lubricant having known properties is utilized.
Four-ball test equipment can also be utilized to formulate the various fluids discussed above. For example, if a fluid includes a component that is no longer available, a fluid including a replacement component can be tested to determine if the new formulation has beneficial or detrimental friction and wear characteristics. Still another use for four-ball testers is to monitor the condition of fluids during use to determine when they are no longer acceptable, or when an additive to the fluid is needed.
As discussed above in connection with FIG. 1, one known type of four-ball tester utilizes a lever arm and weight to generate force on the test balls. Another type of known four-ball tester 15 is illustrated in FIG. 2. The four-ball tester 15 illustrated in FIG. 2 includes a test cup 2, heater block 3, spindle 9, belt 10, electric motor 11, load cell 12, and chain 13 arranged in substantially the same manner as discussed above in connection with the tester 1 of FIG. 1. However, rather than a lever arm 6, the support column 16 of the four-ball tester 15 of FIG. 2 is connected to a pneumatic cylinder (not shown) that pushes upwardly on the support column 16 to apply the test load. The test load of the tester 15 can be adjusted by simply turning the pressure regulator knob to the desired pressure setting. Also, the thrust bearing 4 of tester 15 may be replaced with an air bearing. Such air bearings are utilized in an attempt to provide a very low friction support that provides a more accurate friction reading.
However, existing four-ball testers such as those illustrated in FIGS. 1 and 2 suffer from numerous drawbacks. For example, it is generally not possible to determine the exact actual load on the test balls. The lever type testers may have frictional losses at the fulcrum points, and also in the guide tub. Also, the load arm needs to be adjusted to balance it without any load on it. The load arm must also be adjusted to be perfectly level during testing. These variables can result in inaccurate data if the test setup is not thoroughly checked.
Pneumatic four-ball testers (FIG. 2) also suffer from various drawbacks. For example, the pneumatic loading system needs dry air. Also, the accuracy of the load will be compromised if the pressure meter is not accurate. Also, pneumatic cylinders may “stick-slip” causing a force variation. In an effort to alleviate the stick-slip phenomenon, a free-floating or non-sealing cylinder has been utilized. This arrangement allows air to pass around the piston, between the piston and cylinder wall so that the piston is always loose inside the cylinder. This arrangement provides a constant flow of air through the air cylinder, resulting in various problems. Also, the piston in such free-floating arrangement may angle slightly sideways and lock against the walls of the cylinder.
Furthermore, the air bearing utilized with such four-ball testers may also suffer from various drawbacks. For example, because the air bearing is on top of the air cylinder that applies the load, the pressure of the air bearing needs to be higher than the pressure applied by the air cylinder. This may result in a higher load than expected. Also, air bearings may allow the test cup to tilt slightly, resulting in uneven wear scars and unreliable test results.
Accordingly, a four-ball tester alleviating the problems of existing arrangements would be desirable.